preg_match_all into simple array

I have preg_match_all function:

preg_match_all('#<h2>(.*?)</h2>#is', $source, $output, PREG_SET_ORDER);

It's working as intended, BUT the problem is, it preg_matches all items twice and into a huge multi dimensional array like this for example where it, as intended, preg_matched all 11 items needed, but twice and into a multidimensional array:

Array
(
    [0] => Array
        (
            [0] => <h2>10. <em>Cruel</em> by St. Vincent</h2>
            [1] => 10. <em>Cruel</em> by St. Vincent
        )

    [1] => Array
        (
            [0] => <h2>9. <em>Robot Rock</em> by Daft Punk</h2>
            [1] => 9. <em>Robot Rock</em> by Daft Punk
        )

    [2] => Array
        (
            [0] => <h2>8. <em>Seven Nation Army</em> by the White Stripes</h2>
            [1] => 8. <em>Seven Nation Army</em> by the White Stripes
        )

    [3] => Array
        (
            [0] => <h2>7. <em>Do You Want To</em> by Franz Ferdinand</h2>
            [1] => 7. <em>Do You Want To</em> by Franz Ferdinand
        )

    [4] => Array
        (
            [0] => <h2>6. <em>Teenage Dream</em> by Katie Perry</h2>
            [1] => 6. <em>Teenage Dream</em> by Katie Perry
        )

    [5] => Array
        (
            [0] => <h2>5. <em>Crazy</em> by Gnarls Barkley</h2>
            [1] => 5. <em>Crazy</em> by Gnarls Barkley
        )

    [6] => Array
        (
            [0] => <h2>4. <em>Kids</em> by MGMT</h2>
            [1] => 4. <em>Kids</em> by MGMT
        )

    [7] => Array
        (
            [0] => <h2>3. <em>Bad Romance</em> by Lady Gaga</h2>